1. Field of the Disclosure
The present disclosure relates generally to a remote displacement sensor with many applications. In particular, one application is a strain measuring device used in materials testing. More specifically, the present disclosure relates to the use of visual or optical patterns, including but not limited to moiré patterns, which change in appearance in response to changes in position, and to the methods for detecting and interpreting these changes.
2. Description of the Prior Art
In the prior art, strain measuring devices are well known. Instron, a subsidiary of Illinois Tool Works Inc., makes and sells, among other things, various strain measuring devices. In the past, compressive and tensile properties of materials have been measured by clip-on extensometers that use a resistive strain gauge and, more recently, by non-contact video extensometers. While well-adapted to their intended purposes, clip-on extensometers typically require extensive set-up by trained personnel. Similarly, video extensometers, while well-adapted for their intended purposes, are sensitive to camera motion, air currents, quality of focus and dynamic variations, z-direction motion of the specimen, and displacement of the specimen during gripping, all of which can require compensation to avoid the introduction of errors. Video extensometer applications often require that the centroid of the target dot be measured to an accuracy that represents a small fraction of a camera pixel, thereby requiring sophisticated image processing to achieve necessary sub-pixel accuracy.
Prior art includes U.S. Pat. No. 7,047,819 entitled “Testing of Samples” by Haywood; U.S. Pat. No. 6,164,847 entitled “Image Parameter Detection” to Roy Allen (the present inventor); U.S. Pat. No. 2,787,834 entitled “Grating Strain Gauges” to Shoup; DE 3120653 and EP 0255300.